Conventional LED (light emitting diode) flashlights usually include a DC-DC converter circuit to convert a DC battery voltage from a battery unit into a higher output voltage, and to generate a constant driving current for driving light emission of light emitting diodes. However, over time and with use, internal resistance of the battery unit gradually increases, resulting in gradual decrease of the battery voltage and the output voltage. Under such condition, to maintain the constant driving current may aggravate shortening of battery service life.